1. Field of the Invention
The present invention relates to an optical element holding device configured to hold an optical element such as a collimator lens, an optical element moving device, and an optical pickup apparatus.
2. Description of the Related Art
A common optical pickup apparatus reads/writes information from/to an optical disc by irradiating an information recording layer of the optical disc with a laser beam and receiving the laser beam reflected by the information recording layer, i.e., returning light.
A spherical aberration occurs according to the thickness of a cover layer covering an information recording layer, depending on the standard of an optical disc. Further, different spherical aberrations occur even in one optical disc of a certain standard if the disc has multiple information recording layers, because the position of an information recording layer relative to a cover layer varies depending on which information recording layer is subjected to read/write operations.
An objective lens for CD/DVD has a function of correcting spherical aberration by diffractive annular zones provided in a surface of the lens. However, since an optical disc of the BD standard, for example, employs an objective lens of high NA, it is difficult to correct spherical aberration caused by the objective lens.
To cope with this, such spherical aberration is corrected by moving a collimator lens placed in the midway of the optical path of a laser beam, by a predetermined amount. This feature will be described with reference to FIGS. 10A to 10C. FIGS. 10A and 10B each show a device while cutting away a portion of the device, in which the cut portion is hatched.
A conventional optical element moving device 100 will be described with reference to FIG. 10A, where X directions indicate directions parallel with a laser beam passing through a collimator lens, Y directions indicate directions orthogonal to the X directions on a plane where guide shafts 102 and 103 are placed, and Z directions indicate directions orthogonal to the X directions and the Y directions.
The optical element moving device 100 shown in FIG. 10A mainly includes: an element holding portion 101 configured to hold a collimator lens 104; the guide shaft 103 engaging with a +Y direction-side end portion of the element holding portion 101; the guide shaft 102 inserted in a hole portion located in a −Y direction-side end portion of the element holding portion 101; and a screw-shaped feed shaft 105 configured to move the element holding portion 101 by a predetermined amount.
The feed shaft 105 is a screw-shaped member having a thread groove in its periphery, and a nut 107 made of a resin material is screwed to the feed shaft 105. In addition, plate-shaped nut holding portions 108 and 109 are placed at the −Y direction-side end portion of the element holding portion 101. The nut holding portions 108 and 109 are configured to sandwich the nut 107 from both sides in the X directions with the feed shaft 105 inserted therein.
When the feed shaft 105 is rotated by a drive force of a stepping motor 106 with the above configuration, the nut 107 is moved in conjunction with the rotation. Then, the nut 107 presses any one of the nut holding portions 108 and 109 to move the element holding portion 101 in the X directions by a predetermined amount.
There is a slight play between the feed shaft 105 and the nut 107. In this state, the nut 107 is not fixed to the feed shaft 105 and thus backlash occurs therebetween. Due to this backlash, the position of the collimator lens 104 held by the element holding portion 101 might not be fixed. To solve this, a spring 110 configured to press the element holding portion 101 in the +X direction is inserted on the guide shaft 102 at a position on a −X side of the element holding portion 101. This allows the nut holding portion 108 to press the nut 107 in the +X direction, which removes the play between the feed shaft 105 and the nut 107, suppresses the above-described backlash, and thereby fixes the position of the collimator lens 104.
However, the optical element moving device 100 of the above configuration might cause a failure due to the employment of the nut 107. Specifically, when the nut 107 is made to strike against a −X direction-side or +X direction-side end portion to move the element holding portion 101 back to its initial position, the nut 107 sometimes bites the thread groove of the feed shaft 105 at the end portion and cannot escape therefrom. In addition, the feed shaft 105 is set slightly longer than the movable range of the element holding portion 101 to have enough length to get the element holding portion 101 back to its initial position. With this configuration, the stepping motor 106 keeps applying a driving force to the feed shaft 105 for a while even after the element holding portion 101 is moved to an origin detection position if the element holding portion 101 is placed near the initial position. In this case, the nut 107 keeps pressing the nut holding portions 108 and 109 while this drive force is applied, which makes the nut 107 bite the feed shaft 105.
Further, as shown in FIG. 10B, an upper portion of the nut 107 is covered with a nut holding portion 111. Hence, as shown in FIG. 10C, the nut 107 keeps hitting the nut holding portion 111 while the stepping motor 106 is in normal operation, which results in noise and vibration.
Furthermore, the employment of the nut 107 requires multiple parts such as the nut holding portions 108 and 109 and the spring 110, which complicates assembly work.
A structure using a spring instead of the above nut is disclosed in Japanese Patent Application Publication No. 2010-165445 (Patent Document 1). Referring to FIGS. 7 and 8 of Patent Document 1, the torsion coil spring 80 is attached to the lens holder 62, and a side surface of the torsion coil spring 80 at a position near its leading end portion engages with the groove of the feed screw 63. This configuration enables a mechanism of moving the collimator lens 61 without a nut, and thus avoids the problems caused by the nut.
However, with the invention described in Patent Document 1, the mounting of the torsion coil spring to the optical pickup apparatus is sometimes not easy. Specifically, referring to FIGS. 11A and 11B and their description of Patent Document 1, the torsion coil spring 80 has one end being in contact with the feed screw 63 and the other end fixed by a second press-fix portion 72. Hence, when attached to the optical pickup apparatus, the lens holder 62 needs to be incorporated in the housing of the optical pickup apparatus with such shape of the torsion coil spring 80 kept. This work is very complicated and might reduce work efficiency.
The present invention has been made in view of such circumstances. An objective of the present invention is to provide an optical element holding device in which a spring is in contact with a feed shaft with a simple configuration, an optical element moving device, and an optical pickup apparatus.